UINTACRINUS: ITS STRUCTURE AND RELATIONS. 43 



In No. 76 (PI. IV., Fig. 3) there are traces of the ambulacra only at the 

 lower left corner, where three are visible, evidently at the anterior side, 

 close to the mouth. They cannot be followed, owing to the way the speci- 

 men is crushed. 



It is to be observed that in all these specimens, as well as in some others 

 in which the ambulacra are less plainly visible, the tube always lies pointing 

 away from the mouth. 



Parts of the ambulacra are to be seen on several other specimens, 

 and enough is shown to prove, beyond the slightest doubt, that the 

 position of mouth, ambulacra, and anal tube, as above described, is 

 constant. 



The ambulacral grooves are simply depressions in the perisome. The 

 granules with which they are studded are sometimes smaller than in the 

 rest of the disk, and along the lateral margins of the grooves they are 

 somewhat marked by linear rows, while in the middle they sometimes 

 have a semblance of alternate ari'angement. But both at the sides and 

 in the middle the spicules are of the same irregular shape and spongy 

 texture as in the other parts of the disk. The grooves themselves 

 are broad and shallow, which is probably due to the flattening of the 

 disk by pressure. There is no trace of any covei-ing plates, or any- 

 thing like an ambulacral skeleton ; and from the manner in which 

 the grooves are preserved, it is perfectly evident that there never were 

 any. There is no doubt that every solid structure pertaining to the disk 

 was preserved, just as it was embedded in the soft mud, which formed 

 a firm but exceedingly delicate mould. The plates of the tube are 

 very fragile, yet they are perfectly preserved. The matrix was an im- 

 palpably fine, impure chalk, well adapted to preserve every part in place ; 

 so that it may be regarded as certain that no portion of the calcareous 

 skeleton has been lost. 



It will be well now to consider what other tegmens have been found in 

 fossil Crinoids, with which, as well as with those of living forms, that of 

 Uintacnnns might be compared. 



The tegmen of the Camerata is so widely different, being a solid struc- 

 ture composed of definite plates, with mouth and food grooves rigidly cov- 

 ered, that there is no need of comparing it. 



